Inkjet printing mechanisms use moveable cartridges, also called pens, that use one or more printheads formed with very small nozzles through which drops of liquid ink (e.g., dissolved colorants or pigments dispersed in a solvent) are fired. To print an image, the carriage traverses over the surface of the print medium, and the ink ejection elements associated with the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller. The pattern of pixels on the print media resulting from the firing of ink drops results in the printed image. Certain ink jet inks undergo a process of colorant migration wherein the colorant in the firing chamber is depleted over a short time period. Colorant migration causes print quality defects, especially in documents with fine lines and narrow text characters.
For ink jet inks that undergo a process of colorant migration, delay between firing drops from a nozzle, allows the migration to occur. Thus after a 1.5 second period of non-printing, a drop of black ink can have a shading more like gray than black. After about a 3 second period of non-printing, a drop of black ink can lose most of the colorant and appear almost clear.
The problem of colorant migration is diminished with larger drop volumes. For example, for print cartridges that eject drops that are 30 nanograms (ng) or larger, the large drop weight makes colorant migration less noticeable. However, writing systems that use large drop volumes have significantly worse image quality than those with lower drop weights.
Continuous firing of a print nozzle at high frequency also serves to significantly diminish the effects of colorant migration. For example, the HP Business InkJet 2200 printer, available from Hewlett-Packard Company, having a business address of 3000 Hanover Drive, Palo Alto, Calif. 94304, uses smaller (lower drop weight) 18 ng black drops fired at 36 kilohertz (kHz) from a 600 nozzles per inch (npi) cartridge. However, continuous firing at 36 kHz can cause the printhead to over heat, can cause drop ejection problems and puts constraints on the fluidic architecture design.
In accordance with the preferred embodiment of the present invention, within a printer, compensation is provided for color migration within ink drops. Color compensation is provided by varying firing frequency of each print nozzle so as to fire high frequency bursts of ink drops. Each print nozzle is idle, not being used to eject ink drops, between high frequency bursts of ink drops.